1. Field of the Invention
The present invention relates to a lens including an image blur correction device configured to detect camera shake caused by holding a camera by hand, and to correct the detected camera shake so that the quality of a captured image can be improved. The present invention also relates to a camera, to an optical apparatus including a camera system including the lens and the camera, and to photographing equipment.
2. Description of the Related Art
In the case of recent cameras, all the important operations for shooting, such as exposure control and focus adjustment, can be automated. Thus, even photographers who have little experience in performing camera operations rarely make mistakes in taking photographs.
In addition, since optical apparatuses including a camera and a lens and provided with an image blur correction device (configured to reduce camera shake caused by holding a camera by hand) have been widely used, shooting mistakes caused by photographers rarely occur.
An image blur correction system will be briefly described.
Normally, camera shake caused by hand shake occurring at the time of shooting is a vibration having a frequency in a range from about 1 Hz to about 12 Hz.
In order to reduce the influence of image blur caused by the camera shake, a so-called optical image stabilization system for detecting the camera shake and displacing a correction lens in accordance with a result of the detection has been proposed.
In order to improve the performance of an image-blur correction device, it is necessary, first, to accurately detect shaking of a camera and, second, to properly correct displacement of an optical axis caused by the camera shake.
Such vibration (that is, camera shake caused by hand shake) is detected using a shake detection unit configured to detect angular acceleration, angular velocity, angular displacement, and the like.
Signals output from the shake detection unit are integrated, and an angular displacement signal is calculated. On the basis of the obtained angular displacement signal, the photographing optical axis is made to be off-center, to correct image blurring.
An overview of an image stabilization system including such a shake detection unit will be described with reference to FIG. 9.
FIG. 9 shows an example of a system for suppressing image blur caused by vertical camera shake 81p and horizontal camera shake 81y, which are indicated by arrows 81. In the example shown in FIG. 9, an image blur correction device is provided in an interchangeable lens of a single-lens reflex camera.
Referring to FIG. 9, the system includes a lens barrel 82, a vertical camera shake detection unit 83p, and a horizontal camera shake detection unit 83y. The vertical camera shake detection unit 83p and the horizontal camera shake detection unit 83y detect shake in directions denoted by 84p and 84y, respectively.
The system also includes a correction optical device 85. The correction optical device 85 includes coils 86p and 86y for applying driving force to the correction optical device 85 and position detecting elements 87p and 87y for detecting the position of the correction optical device 85. A position control loop provided in the correction optical device 85 is driven in accordance with outputs of the shake detection units 83p and 83y as set points. Accordingly, stabilization in an image plane 88 can be ensured.
In addition, an image blur correction device described in Japanese Patent Laid-Open No. 7-218967 includes a mechanical mechanism for mechanically holding a correction optical system at the center of correction when the correction optical system is not driven.
An image blur correction device in which a correction optical system is held at the center of correction by using elastic members, such as springs, instead of using the mechanical mechanism, is also available. Since such an image blur correction device does not include a mechanical holding unit, a small-size image blur correction device can be achieved.
In a shake correction device described in Japanese Patent Laid-Open No. 8-184870, when an image blur correction operation is not performed, a correction lens is held by springs. Thus, when a shock is applied to the springs, the springs resonate at resonance frequencies. Depending on the spring constant of a spring, the resonance amplitude may significantly increase.
In the case of shooting with a single-lens reflex camera, mirror driving and shutter driving are performed at the time of shooting. In the case that a shock caused by such driving is applied to the above-described image blur correction device, the correction lens can resonate. This resonance may affect a captured image.
In order to avoid such an unwanted resonance, electric power is supplied to driving coils so that the correction lens can be electrically held so as to be centrally aligned with the optical axis. The correction lens may be held in the vicinity of the optical axis to an extent that allows the influence to be exerted on a captured image to become negligible.
In order to attenuate an unwanted resonance more effectively, the correction lens must be electrically held in accordance with a driving characteristic having an increased damping. However, in the case of a characteristic having an increased damping, the phase lag in image blur correction increases and the performance of image blur correction is thus degraded.
FIGS. 10A and 10B are Bode diagrams each representing a difference in frequency characteristic with respect to a difference in damping. In each of FIGS. 10A and 10B, a characteristic represented by a solid line is a frequency characteristic in the case of a low damping and a characteristic represented by a dotted line is a frequency characteristic in the case of a high damping.
As is clear from the gain characteristic represented by the solid line shown in FIG. 10A, a large resonance gain is indicated at the resonance frequency. Thus, in the case that a vibration at the resonance frequency is input into the image blur correction device, the correction lens largely resonates. Thus, such a large resonance may affect a captured image.
In the case that the damping is increased in order to achieve a smaller resonance gain, the characteristic represented by the dotted line can be achieved. However, a large phase lag is generated at a hand-shake frequency range around 10 Hz. As a result, the performance of image blur correction is degraded.
In addition, unwanted resonance caused by the above-mentioned shock is attenuated with time. Thus, in the case where the shutter speed of a camera is fast, a large influence of unwanted resonance is exerted on a captured image. On the other hand, in the case where the shutter speed is slow, a small influence of unwanted resonance is exerted on a captured image.